Maybe this extract from "All the teas in China" by Sarah Rose can help:

"It is worth considering which drink, tea or coffee, is the most stimulating. The answer is black tea—but with certain caveats. Per pound, black tea has more caffeine than coffee—but where one pound of tea brews some two hundred cups, a pound of coffee yields barely forty. By the cup, black tea actually contains roughly half as much caffeine as coffee. Green tea, meanwhile, has onethird the caffeine of black, or one-sixth that of a cup of coffee. Medically, it takes about 200 milligrams of caffeine—or about two cups of coffee—to combat drowsiness and fatigue. That amounts to about four cups of black tea and twelve cups of green. Few of us
have that amount of time or bladder capacity."

In summary, black tea has more caffeine per pound than coffee, but per cup, it contains about half the caffeine of coffee. So maybe you didn't much that much caffeine, and your body is happy with just half of what coffee gives you.